Generate a v0.16 apply plan for the listeners migrate_v016.py leaves behind

First piece of ARCHITECTURE.md 4.3's apply-plan, and the piece that decides
whether a migrated server answers at all: server.listener is not among the
settings the official converter carries, so a freshly migrated instance
binds nothing. Every other unmigrated setting degrades the server; this one
stops it being a server.

internal/applyplan maps server.listener.* onto x:NetworkListener objects and
reports its own coverage. Against the smoke instance that is 24 of 3,505
unmigrated keys - 0.7% - and the output says 0.7%, listing the largest
groups it did not touch. A plan covering a fraction while implying
completeness would be worse than no plan.

The wire format was confirmed against the binary, not the documentation.
The published schema reference gives NetworkListener.bind as a JSON array;
0.16.14 rejects that outright ("Invalid value for object property.
Properties: bind"). The encoding it accepts is a value-keyed set,
{"[::]:25": true}, found by applying a plan to a live recovery-mode 0.16.14
and reading it back with `stalwart-cli snapshot`. Only mappings confirmed
that way are in DefaultGenerators; managesieve -> manageSieve is the one
protocol whose spelling changes, and an unrecognized protocol is reported
and skipped rather than passed through to fail at apply time.

Operations are upserts matched on name, so a plan can be re-run - an
operator will run it more than once - and the supplement is applied after
export.json rather than merged into it, so a generated mapping can never
override one the official script got right.

Verified end to end: rehearse against a real 0.15.5 generated ten
listeners, `stalwart-cli apply` created all ten on a real 0.16.14 with zero
failures, a snapshot read them back with correct protocols, binds and TLS
flags, and re-applying reported 10 updated / 0 created / 0 failed.

internal/backup/settingsdump.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
@@ -263,3 +264,51 @@ func ReadUnmigratedReport(path string) (*UnmigratedReport, error) {
 	sort.Slice(report.Prefixes, func(i, j int) bool { return report.Prefixes[i].Keys > report.Prefixes[j].Keys })
 	return report, nil
 }
+
+// ReadSettingsDump loads the flat {key: value} settings map
+// migrate_v016.py's dump step writes.
+func ReadSettingsDump(path string) (map[string]string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("backup: read settings dump %s: %w", path, err)
+	}
+	var raw map[string]any
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return nil, fmt.Errorf("backup: parse settings dump %s: %w", path, err)
+	}
+	settings := make(map[string]string, len(raw))
+	for k, v := range raw {
+		if s, ok := v.(string); ok {
+			settings[k] = s
+			continue
+		}
+		settings[k] = fmt.Sprint(v)
+	}
+	return settings, nil
+}
+
+// ReadUnmigratedKeys returns the set of settings keys migrate_v016.py
+// reported it did not carry over.
+//
+// unmigrated.txt lists prefixes and counts rather than individual keys, so
+// this expands those prefixes against the settings dump. That is why it
+// needs both files: the report says "spam-filter.rule: 424 keys", and only
+// the dump knows which 424.
+func ReadUnmigratedKeys(reportPath string, settings map[string]string) (map[string]bool, error) {
+	report, err := ReadUnmigratedReport(reportPath)
+	if err != nil {
+		return nil, err
+	}
+	keys := map[string]bool{}
+	if report == nil {
+		return keys, nil
+	}
+	for _, p := range report.Prefixes {
+		for k := range settings {
+			if k == p.Prefix || strings.HasPrefix(k, p.Prefix+".") {
+				keys[k] = true
+			}
+		}
+	}
+	return keys, nil
+}